s/all built-in definitions/all available definitions/

Suggested by mislav: https://github.com/rbenv/ruby-build/pull/1402#discussion_r389060742

> I wonder should we substitute "built-in" with something like "all
> available definitions". Since ruby-build will additionally pick up
> definitions found under the paths listed in RUBY_BUILD_DEFINITIONS,
> these definitions will get listed even if they are not "built-in".
> Therefore, "built-in" feels like a misnomer here.